Obviously teenager is 13-19.
“Young adult” would start at 20, but where’s the cutoff at the upper end? Similarly, what’s the range for “adult”, “old”, “elderly”, " ancient"?
If someone asks for responses from “old men”, how do I know if it applies to me?
It’s slightly variable and largely depends on the dragon’s subspecies, although 5e removed all that in lieu of a simpler, but more boring, universal scale that covers all of them equally.